It is recalled that an airborne radar comprises a transmitter which emits signals, more precisely radio waves. The latter are reflected by the target and detected by a receiver, also incorporated within the radar. The position of the targets lying in the field of the emitted radio waves is estimated by virtue of the return time of the reflected waves.
More precisely, the position of a target is deduced from the attitude of the aerial transporter and the angle of pointing of the radar antenna. By “the attitude of the aerial transporter” is meant the values taken by its roll and pitch angles. These values are generally obtained with the aid of sensors such as gyroscopes, acceleration sensors and angular rate sensors.
Within the framework of certain applications, it is very important to reduce the uncertainties relating to the radar antenna beam pointing angle. Now, the aforesaid sensors give rise to fixed and dynamic errors in the measurements of the roll and pitch angles.
Other inevitable errors are related to the installation of the radar on the aerial transporter, to the inertial platform, to the deformation of the structure of the aircraft or of the materials of the structure forming the support of the pedestal of the radar (situated in a non-pressurized environment).
Errors are also related to the positioning of the beam of the radar antenna itself, due for example:
to the installation of the radar on the transporter,
to the deformation of the structure of the aircraft or of the materials which constitute the support of the pedestal of the radar, or else
to the calibration of the radar beam itself.
By way of example, an error of 1 degree in the angle of the radar beam gives rise to an error of 6500 m of altitude in the location of a target, at 200 NM.
A scheme for compensating for the pitch angle error is known from the document U.S. Pat. No. 5,485,156. This comprises two sightings with the aid of the radar beam, for two angles of elevation that are close together in elevation, an identification of the shape of the antenna pattern with the aid of the signal reflected by the ground. The difference between the position of the ground and the angle of elevation measured by the sensor then gives the error to be compensated.
Nevertheless, because of the use of an external sensor (radioaltimeter) to accurately ascertain the altitude and because of the disregarding of the relief of the ground and of the curvature of the earth, numerous errors appear. The scheme is therefore rather unreliable.
Moreover, the estimation of the position of the antenna pattern on the basis of only two elevational pointings can lead to errors with respect to a device that uses more elevational pointings.
Finally, the scheme proposed by the document U.S. Pat. No. 5,485,156 disregards the presence of possible disturbances (meteorological phenomena: cloud, hail, etc., other aircraft, etc.), which is detrimental to the compensation for the angle of elevation error.
The invention is aimed notably at solving these problems.